A horizontal type continuous casting machine for manufacturing a cast steel strand is now industrialized, in which machine a cast steel strand is horizontally and intermittently withdrawn from a horizontal mold fitted to the lower portion of the side wall of a tundish containing molten steel, by a plurality of cycles each comprising a pull and a push.
The above-mentioned conventional horizontal type continuous casting machine is described below with reference to FIG. 1. In FIG. 1, 1 is a tundish for containing molten steel 11 from a ladle (not shown); 2 is a horizontal mold having a cooling water passage 2A in the wall thereof. The horizontal mold 2 is horizontally fitted, through a feed nozzle 3 and a break ring 4, to an opening 1A formed in the lower portion of the side wall of the tundish 1, and is cooled by cooling water supplied into the cooling water passage 2A. In FIG. 1, 5 is a cast steel strand having a square cross-sectional shape, which has been horizontally and intermittently withdrawn by a plurality of pinch rolls described later from the horizontal mold 2; 5A is a solidified shell of the cast steel strand 5; 6 is a cooling zone provided following the horizontal mold 2 on the same horizontal level as that of the horizontal mold 2; 7 are a plurality of spray nozzles provided in the cooling zone 6, the plurality of spray nozzles 7 spraying cooling water from above and below the cast steel strand 5 onto the surface of the solidified shell 5A of the cast steel strand 5 passing through the cooling zone 6 to cool the cast steel strand 5; and 8 are a plurality of pinch rolls for horizontally and intermittently withdrawing the cast steel strand 5 from the horizontal mold 2 by a plurality of cycles each comprising a pull and a push.
The cast steel strand 5 is manufactured as follows using the above-mentioned conventional horizontal type continuous casting machine. Molten steel 11 contained in the tundish 1 flows through the feed nozzle 3 and the break ring 4 into the horizontal mold 2 and is cooled therein. The cast steel strand 5 thus cast is horizontally and intermittently withdrawn from the horizontal mold 2 by the pinch rolls 8. Then, the cast steel strand 5 is cooled by cooling water ejected from the plurality of spray nozzles 7 provided in the cooling zone 6 onto the surface of the solidified shell 5A of the cast steel strand 5 passing through the cooling zone 6. Then, the cast steel strand 5 is completely solidified to the core by natural cooling. The cast steel strand 5 is thus continuously manufactured.
An example of the above-mentioned cycle comprising a pull and a push which is applied to the cast steel strand 5 is illustrated in FIG. 2. In FIG. 2, the abscissa shows time, and the ordinate indicates the pulling speed of the cast steel strand 5 in the upper half starting from the point 0, and the compressive force applied to the cast steel strand 5 by a push of the cast steel strand 5 in the lower half starting from the above mentioned point 0.
FIGS. 3(A) and 3(B) illustrate partial sectional views of the formation of the solidified shell 5A of the cast steel strand 5 in the horizontal mold 2 when horizontally and intermittently withdrawing the cast steel strand 5 in consecution from the horizontal mold 2. FIG. 3(A) is a partial sectional view illustrating the formation of a solidified shell 5'A of the cast steel strand 5 during the pull in one cycle comprising a pull and a push of the cast steel strand 5, whereas FIG. 3(B) is another partial sectional view illustrating the formation of the solidified shell 5'A of the cast steel strand 5 upon the completion of the above-mentioned one cycle.
The cast steel strand 5 is horizontally pulled from the horizontal mold 2 at a prescribed pulling speed for a prescribed pulling time. Then, the cast steel strand 5 is pushed back in the direction opposite to the pulling direction for a prescribed pushing time. This pushing back prevents the solidified shell 5A of the cast steel strand 5 from being broken due to heat contraction in the horizontal mold 2 and a breakout from occurring as a result.
However, an incompletely welded portion known as a cold shut 9 is formed on the juncture face between two adjacent portions 5'A and 5"A of the solidifed shell 5A of the cast steel strand 5, which adjacent portions 5'A and 5"A are formed by a plurality of cycles each comprising a pull and a push.
The cause of the formation of the above-mentioned cold shut 9 is as follows. As shown in FIGS. 3(A) and 3(B), the solidifed shell 5'A of the cast steel strand 5 formed during one cycle comprising a pull and a push is cooled from three sides including not only the side facing the horizontal mold 2 but also the side facing the break ring 4 and the side facing the solidified shell 5"A of the cast steel strand 5 which has already been formed in the cycle precedent to that one cycle. As a result, solidification of the solidified shell 5'A of the cast steel strand 5 begins on the three sides mentioned above. The structure of the solidified shell 5'A which contacts with the solidified shell 5"A of the cast steel strand 5 grows horizontally and in the direction opposite to that of the structure of the solidified shell 5"A of the cast steel strand 5. As a result, the structure of the juncture face between the solidified shells 5'A and 5"A of the cast steel strand 5 becomes discontinuous so that a complete welding is not achieved at this juncture face. This causes occurrence of a cold shut 9.
The above-mentioned cold shut 9 poses no problem if it is completely welded, but when not welded completely, cracks are produced along the cold shut 9 at the time of thermal contraction of the solidified shell 5A of the cast steel strand 5 caused by cooling in the cooling zone 6. These cracks become fine cold shut cracks. When the cast steel strand 5 having these fine cold shut cracks is rolled, these fine cold shut cracks remain on the surface portion of the resultant rolled product, thus remarkably deteriorating the quality of the rolled product.
For the removal of these fine cold shut cracks the following method is conceivable: shortening the time of one cycle comprising a pull and a push of the cast steel strand 5 to inhibit growth of the structure of the cold shut 9 and maintaining the temperature of the solidified shell 5A of the cast steel strand 5 on a high level, to thereby improve the weldability of the cold shut 9.
To shorten the time of a cycle however requires a control technique of accomplishing the cycle of a pull and a push at a high accuracy, and is consequently difficult to practice. Even when such a control technique is developed, for example in the case of a cast steel strand having a square cross-section, the four corners of the cast steel strand are over-cooled as compared with the other portions. It is therefore impossible to substantially completely remove the fine cold shut cracks.
Also in the horizontal type continuous casting method comprising pulls and pushes of the cast steel strand 5 as mentioned above, the cold shut 9 can be welded by increasing the pushing force of the cast steel strand 5 to some extent. However, if an excessive pushing force is applied to the cast steel strand 5, buckling occurs in the solidified shell 5A of the cast steel strand 5, resulting in a breakout of unsolidified molten steel.
A method for solving the above-mentioned problem is disclosed in Japanese Patent Provisional Publication No. 128255/83 (hereinafter referred to as the "prior art"). The prior art is described below with reference to FIG. 4.
In FIG. 4, 10 is a ladle; 1 is a tundish containing molten steel 11 from the ladle 10; 12 is a teeming nozzle fixed vertically to an opening 1A of the bottom wall of the tundish 1; 2' is a vertical mold provided directly below the tundish 1; 5 is a cast steel strand vertically downwardly withdrawn continuously from the vertical mold 2' by a plurality of pinch rolls 8; 5A is a solidified shell of the cast steel strand 5; 6 is a cooling zone provided following the vertical mold 2' on the same vertical level as that of the vertical mold 2', a plurality of spray nozzles (not shown) for ejecting cooling water onto the solidified shell 5A of the cast steel strand 5 withdrawn from the vertical mold 2' being provided in the cooling zone 6; 13 is a shooting zone provided following the cooling zone 6 on the same vertical level as that of the cooling zone 6; and 14 are a plurality of shooters provided in the shooting zone 13, each of the plurality of shooters 14 continuously ejecting a plurality of metal shots 15 onto the surface of the solidified shell 5A of the cast steel strand 5.
The plurality of metal shots 15 discharged from the shooting zone 13 after ejection onto the surface of the solidified shell 5A of the cast steel strand 5 are supplied in recycle into the plurality of shooters 14 in the shooting zone 13 by a metal shot recycling means.
The above-mentioned metal shot recycling means comprises: a metal shot discharge pipe 16 for transporting the plurality of metal shots 15 discharged from the shooting zone 13 after ejection onto the surface of the solidified shell 5A of the cast steel strand 5 to a conveyor described later, the metal shot discharge pipe 16 being downwardly inclined toward the downstream end thereof; a conveyor 17 for transporting the plurality of metal shots 15 discharged through the metal shot discharge pipe 16 to a hopper described later, the conveyor 17 being provided substantially vertically at the downstream end of the metal shot discharge pipe 16; a hopper 18 for storing the plurality of metal shots 15 received from the conveyor 17 and supplying same to the plurality of shooters 14 in the shooting zone 13, the hopper 18 being provided above the shooting zone 13; and a metal shot supply pipe 19 for communicating the hopper 18 with the shooting zone 13, the metal shot supply pipe 19 supplying the plurality of metal shots 15 through respective branch pipes 19A into the plurality of shooters 14 in the shooting zone 13.
Now, the shooter 14 is described with reference to FIG. 5. In FIG. 5, 20 is a vertical disk; 21 is a motor for rotating the vertical disk 20 at a high speed; 22 are a plurality of impellers radially and vertically fixed to one surface of the vertical disk 20; 23 is a cover for covering the vertical disk 20 and the plurality of impellers 22; 24 is a shooting port formed at a portion of a peripheral wall 23A of the cover 23; and 25 is a valve fitted to each of the respective branch pipes 19A of the metal shot supply pipe 19 for regulating the flow rate of the plurality of metal shots 15.
The branch pipe 19A of the metal shot supply pipe 19 communicates with the central portion of one of vertical side walls 23B of the cover 23. When the plurality of metal shots 15 in the hopper 18 are supplied to the rotational center portion of the plurality of impellers 22 rotating at a high speed through the branch pipe 19A of the metal shot supply pipe 19, the plurality of impellers 22 continuously eject at a high speed the plurality of metal shots 15 from the shooting port 24 onto the surface of the solidified shell 5A of the cast steel strand 5.
The plurality of metal shots 15, made of steel or cast iron, have a diameter of from about 0.5 mm to about 3.5 mm.
In FIG. 4, the branch pipe 19A of the metal shot supply pipe 19 and the metal shot discharge pipe 16, which are illustrated for only one of the two shooters 14, are of course fitted also to the other shooters 14.
As shown in FIG. 4, fine cold shut cracks on the surface portion of the solidified shell 5A of the cast steel strand 5 are removed by the above-mentioned prior art as follows. The cast steel strand 5 vertically downwardly withdrawn continuously from the vertical mold 2' by the plurality of pinch rolls 8 is cooled in the cooling zone 6 by cooling water ejected from the plurality of spray nozzles not shown onto the surface of the solidified shell 5A of the cast steel strand 5 passing through the cooling zone 6. Then, the plurality of metal shots 15 are continuously ejected in the shooting zone 13 from the plurality of shooters 14 onto the surface of the solidified shell 5A of the cast steel strand 5 passing through the shooting zone 13. Fine cold shut cracks, which have been produced on the surface portion of the solidified shell 5A of the cast steel strand 5 withdrawn from the vertical mold 2', are welded and removed under the pressure applied by the plurality of metal shots 15 thus ejected. The plurality of metal shots 15 discharged from the shooting zone 13 after ejection onto the surface of the solidified shell 5A of the cast steel strand 5 in the shooting zone 13 are transported through the metal shot discharge pipe 16 to the conveyor 17, and then transported on the conveyor 17 to the hopper 18 for use in recycle.
According to the prior art described above, it is possible to remove the fine cold shut cracks of the cast steel strand manufactured by a vertical type continuous casting machine. Direct application of this prior art to the above-mentioned horizontal type continuous casting machine allows removal of the fine cold shut cracks of a horizontally and continuously cast steel strand.
However, the above-mentioned prior art involves the following problems. In the prior art, since the shooting zone 13 is provided immediately following the cooling zone 6, the plurality of metal shots 15 are ejected onto the surface of the solidified shell 5A of the cast steel strand 5 immediately after cooling in the cooling zone 6. However, ejection of the plurality of metal shots 15 onto the surface of the solidified shell 5A of the cast steel strand 5 which has been hardened by cooling in the cooling zone 6 does not permit substantially complete welding and removal of the cold shut cracks which have been produced in the surface portion of the solidified shell 5A of the cast steel stand 5, under the pressure applied by the plurality of metal shots 15.
For these reasons, there is a strong demand for the development of a method and an apparatus which permit substantially complete welding and removal of the fine cold shut cracks of a horizontally and continuously cast steel strand, with the use of ejection of a plurality of metal shots, but such method and apparatus have not been proposed as yet.